Spherical microphone arrays, including those that are rotationally symmetric, can offer virtually any spatial directivity and are thus attractive in various applications such as beamforming, speech enhancement, spatial audio recordings, sound-field analysis, and plane-wave decomposition. Two spherical microphone array configurations are commonly employed. The sphere may exist physically, or may merely be conceptual. In the first configuration, the microphones are arranged around a rigid sphere (e.g., made of wood or hard plastic or the like). In the second configuration, the microphones are arranged in a free-field around an “open” sphere, referred to as an “open-sphere configuration.” Although the rigid-sphere configuration provides a more robust numerical formulation, the open-sphere configuration might be more desirable in practice at low frequencies, where large spheres are realized.
In open-sphere configurations, most practical microphones have a drum-like or disc-like shape. In practice, it would be desired to move the capsules closer to the center of the array in order to maintain the directional performance of the array up to the highest audio frequencies. So for microphones of a given size, the gap between adjacent microphones will become smaller as they are pulled in, perhaps to the point where adjacent microphones touch.
This situation worsens when directional microphones (i.e., microphones having an axis along which they exhibit maximum sensitivity) are employed, as directional microphones are commonly much bulkier than omnidirectional microphones (i.e., microphones having a sensitivity independent of the direction). An exemplary type of directional microphone is called a shotgun microphone, which is also known as a line plus gradient microphone. Shotgun microphones may comprise an acoustic tube that by its mechanical structure reduces noises that arrive from directions other than directly in front of the microphone along the axis of the tube. Another exemplary directional microphone is a parabolic dish that concentrates the acoustic signal from one direction by reflecting away other noise sources coming from directions other than the desired direction.
A sound capture system that avoids the dimensional problems noted above, particularly with an open-sphere microphone array, is desired.